Compact magnetic fluid low pressure seal

ABSTRACT

A bearing and magnetic fluid seal assembly includes a shaft mounted by axially spaced-apart bearings for rotation relative to a housing. A magnetic fluid seal includes an annular magnet axially positioned between the bearings in the space that is wasted and not utilized for any purpose, and a thin, disk-like annular polepiece positioned between one of the bearings and the environment to be protected by the seal. Magnetic flux from the magnet is conducted by a magnetically permeable housing or shaft to an annular gap between the inside diameter of the polepiece and the shaft. A magnetic fluid is retained in the gap by the magnetic flux, thereby providing an exclusion seal. Embodiments utilizing radially and axially polarized magnets are provided. A magnetic fluid seal can be provided at each end of the assembly. The bearings supporting the shaft can be magnetically permeable or nonmagnetic. The seal(s) can be electrically conducting or nonconducting depending upon the type of ferrofluid used.

FIELD OF THE INVENTION

This invention relates to a magnetic fluid seal assembly and, moreparticularly, to a compact magnetic fluid seal assembly with a shortaxial length which is particularly useful in a computer magnetic diskdrive. The seal may be electrically conducting or nonconducting,depending upon the type of ferrofluid used.

BACKGROUND OF THE INVENTION

In modern disk drive assemblies, the mechanical clearance between thesurface of the magnetic disk which stores data and the read/write headwhich transfers data to and from the disk is extremely small, i.e.,about 10 microinches. Any particulate contamination on the disk surfaceor in the region of the read/write head is likely to cause a malfunctionof the assembly. Consequently, the computer disk and the head must beenclosed and must be protected against contamination from the outsideenvironment. The motor which drives the disk shaft and the bearingswhich support the disk shaft are sources of contamination due to theirmechanically movable parts. However, it is generally inconvenient toenclose the motor. Thus, a magnetic fluid exclusion seal is typicallyemployed on the drive shaft to prevent contaminants from the motor andthe bearings and from the external environment from reaching theinterior of the disk assembly along the drive shaft.

The trend toward miniaturization of disk drives has required thinner andthinner drives, thereby severely limiting the axial space available formagnetic fluid seals and bearings, and requiring designs with a verysmall axial length or thickness. Magnetic fluid seals for use in smalldisk assemblies such as 51/4 inch and 31/2 inch spindles are typicallyrequired to have an axial thickness on the order of 1 mm, while diskshaft bearings are typically required to have an axial thickness on theorder of 5 mm.

A variety of bearing assemblies and separate magnetic fluid seals toperform the exclusion function have been disclosed in the prior art.Although such configurations function satisfactorily, they do not meetthe seal thickness requirements of present disk drives. For example,U.S. Pat. No. 4,293,137, issued Oct. 6, 1981 to Ezekiel, discloses aseal configuration wherein annular magnets are positioned at oppositeends of a bearing with a magnetic fluid seal in the gap between eachannular magnet and the shaft. This configuration is not suitable for usein applications requiring a very small axial dimension such as the diskdrive assembly described above, since a reduction in magnet thicknesswould reduce the magnetic field below the level required for holding themagnetic fluid in the seal area. Bearing and seal assemblies wherein aseal is axially positioned between two bearings which support a shaftare disclosed in U.S. Pat. No. 4,526,380, issued July 2, 1985 to Raj etal and U.S. Pat. No. 4,407,518, issued Oct. 4, 1983 to Moskowitz et al.Although the disclosed seals do not add to the axial length of theassembly, the seals do not exclude contaminants generated by the bearingadjacent to the disk area.

It a general object of the present invention to provide improvedmagnetic fluid seal assemblies.

It is another object of the present invention to provide magnetic fluidseal assemblies with a very small axial dimension.

It is a further object of the present invention to provide magneticfluid seal assemblies which are easily manufactured and which are low incost.

SUMMARY OF THE INVENTION

According to the present invention, these and other objects andadvantages are achieved in a magnetic fluid seal assembly comprising ahousing, a shaft having an axis of rotation, a bearing assembly forsupporting the shaft and for permitting rotation of the shaft relativeto the housing, and a magnetic fluid seal. The bearing assembly includesat least two axially spaced-apart bearings between the shaft and thehousing. The magnetic fluid seal includes magnet means for producing amagnetic field axially positioned between the bearings in a space thatremains generally unutilized and further includes polepiece meansaxially positioned outside the bearing assembly between the bearingassembly and the environment to be protected by the seal. The polepiecemeans includes an annular polepiece extending inwardly from the housingtoward the shaft, and having a closely-spaced, noncontactingrelationship with the shaft, thereby defining a gap between thepolepiece and the shaft. A magnetic fluid is retained by the magneticfield in the gap between the polepiece and the shaft to provide anexclusion seal. Either the housing or the shaft, or in some cases both,are magnetically permeable so that at least a portion of the magneticflux is conducted between the magnet means and the gap between thepolepiece and the shaft. Preferably, the annular polepiece is a thin,annular disk and the seal is formed on the shaft between the bearing andthe disk enclosure, or other environment to be protected, without addingappreciably to the axial dimension of the assembly.

Included within the scope of the invention are configurations withradially and axially polarized magnet means positioned between thebearings. In addition, annular polepieces can be positioned outside thebearing assembly at each end for additional sealing. The magnetic fieldcan be provided by a single annular magnet or by a pair of spaced-apartannular magnets between the bearings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention together with otherand further objects, advantages and capabilities thereof, reference ismade to the accompanying drawings which are incorporated herein byreference and in which:

FIG. 1 is a cross-sectional view of a bearing and magnetic fluid sealassembly in accordance with the prior art;

FIG. 2 is a cross-sectional view of a bearing and magnetic fluid sealassembly in accordance with the present invention utilizing a radiallypolarized magnet attached to the housing;

FIG. 3 is a cross-sectional view of a bearing and magnetic fluid sealassembly in accordance with the present invention utilizing a radiallypolarized magnet attached to the shaft;

FIG. 4 is a cross-sectional view of a bearing and magnetic fluid sealassembly in accordance with the present invention utilizing an axiallypolarized magnet attached to the housing;

FIG. 5 is a cross-sectional view of a bearing and magnetic fluid sealassembly in accordance with the present invention utilizing an axiallypolarized magnet attached to the shaft;

FIG. 6 is a cross-sectional view of a bearing and magnetic fluid sealassembly in accordance with the present invention utilizing a radiallypolarized magnet attached to the housing and polepieces at opposite endsof the bearing assembly;

FIG. 7 is a cross-sectional view of a bearing and magnetic fluid sealassembly in accordance with the present invention utilizing a radiallypolarized magnet attached to the shaft and polepieces at opposite endsof the bearing assembly;

FIG. 8 is a cross-sectional view of a bearing and magnetic fluid sealassembly in accordance with the present invention utilizing a pair ofaxially polarized magnets attached to the housing and a pair ofpolepieces at opposite ends of the bearing assembly; and

FIG. 9 is a cross-sectional view of a bearing and magnetic fluid sealassembly in accordance with the present invention utilizing a pair ofaxially polarized magnets attached to the shaft and a pair of polepiecesat opposite ends of the bearing assembly.

DESCRIPTION OF THE PRIOR ART

A computer disk drive assembly in accordance with the prior art is shownin FIG. 1. One or more magnetic disks 10 are attached at their center toa spindle shaft 12. The shaft 12 and the magnetic disk 10 are caused torotate about an axis 14 by a motor 16. Information is transferred to andfrom the disks 10 by a disk head 18. A housing 20 supports the motor 16and a pair of bearings 22, 24 which are spaced along and support theshaft 12. The space between the two bearings 22, 24 is empty and notutilized for any purpose. The disks 10 are protected againstcontamination by a cover enclosure 26. Positioned along the shaft 12between upper bearing 22 and the interior of cover enclosure 26 is amagnetic fluid seal 28 which typically includes an annular magnet 30 andannular polepieces 31, 32 at opposite ends of magnet 30. The magnet 30and polepieces 31, 32 are attached to housing 20 and are dimensioned sothat gaps are defined between the inner edges of polepieces 31, 32 andshaft 12. A magnetic fluid 33 is retained in the gaps by the magneticfield of magnet 30. The seal 28 excludes contaminants generated bybearings 22, 24, motor 16 and by the external environment from reachingthe interior of enclosure 26. The drawback to the configuration shown inFIG. 1 is that the magnetic fluid seal 28 adds directly to the axiallength of the assembly and typically has an axial dimension on the orderof 3.8 mm.

DETAILED DESCRIPTION OF THE INVENTION

A computer disk drive assembly incorporating a bearing and magneticfluid seal assembly in accordance with the present invention is shown inFIG. 2. One or more magnetic disks 40 are attached at their center to aspindle shaft 42 and are enclosed by a cover enclosure 43. The shaft 42and the magnetic disks 40 are caused to rotate about an axis 44 by amotor 46 attached to the shaft 42. Information is transferred to andfrom the disks 40 by a disk head 48. A housing 50 supports the motor 46and a bearing assembly comprising a pair of bearings 52, 54. Thebearings 52, 54 are axially spaced apart by a sufficient distance tosupport the shaft 42 for rotation about the axis 44. The spacing can bedetermined by a cylindrical spacer (not shown) or by shoulders 56 onhousing 50. Typically, bearings 52, 54 are ball bearings with a shortaxial length. However, any suitable bearing can be utilized. An annularspace 58 between shaft 42 and housing 50 is closed at its ends bybearings 52, 54.

In accordance with the present invention, a compact magnetic fluidexclusion seal is formed by an annular magnet positioned in the space 58between bearings 52 and 54, at least one polepiece axially positionedbetween bearing 52 and the environment to be protected againstcontamination, and a magnetic fluid in a gap between the polepiece andthe shaft. Either the housing 50 or the shaft 42 is made of amagnetically permeable material to conduct magnetic fields from themagnet to the polepiece. The magnetic fluid is retained in the gapbetween the polepiece and the shaft by the magnetic field. The bearingand magnetic fluid seal assembly of the present invention can beutilized with a fixed housing and rotating shaft or with a fixed shaftand rotating housing. The preferred embodiments of the invention aredescribed hereinafter with reference to FIGS. 2-9.

Referring again to FIG. 2, a radially polarized annular magnet 60 isattached to housing 50 and has sufficient inside diameter to define agap between the magnet 60 and shaft 42. The magnet 60 is located withinthe annular space 58, preferably in relatively close proximity to thebearing 52. An annular polepiece 62 having a thin, disk-like shape isattached at its outside diameter with a fixed seal to housing 50 and islocated at the opposite end of bearing 52 from magnet 60 between bearing52 and cover enclosure 43. The polepiece 62 has a closely-spaced,noncontacting relationship with shaft 42 so that a gap is definedbetween the inside diameter of polepiece 62 and the shaft 42. In theembodiment of FIG. 2, the housing 50, at least in the region between theannular magnet 60 and the polepiece 62, is made of a magneticallypermeable material. The shaft 42 can be made of either magneticallypermeable or nonmagnetic material. A magnetic field from the magnet 60is conducted through the magnetic housing 50 and magnetic polepiece 62to the gap between the polepiece 62 and the shaft 42. A magnetic fluid64 is retained by the magnetic field in the annular gap between thepolepiece 62 and the shaft 42, thereby forming a low pressure exclusionseal while permitting rotation of the shaft 42 relative to the housing50.

The magnet 60 can be of low energy rigid or flexible ferrite type. Thepolepiece 62 is preferably a thin disk of magnetically permeablematerial but can have a relatively thick axial dimension if spacepermits. The clearance between the shaft 42 and the inside diameter ofmagnet 60 is large enough to divert magnetic flux through housing 50 topolepiece 62 so that the magnetic field retains magnetic fluid in thegap between the polepiece 62 and the shaft 42. In another feature of theinvention, an electrically conducting magnetic fluid 64 can be utilizedto conduct static charge from the disks 40 through shaft 42, polepiece62 and housing 50, thereby grounding the rotating disks. The pole pieceouter diameter in this case is electrially connected (with silver paint,conducting adhesive, etc.) with the housing.

In all the embodiments (FIGS. 2-9), the bearings 52 and 54 can be eithermagnetically permeable or nonmagnetic. If the bearings are magneticallypermeable, some magnetic flux loss will occur through the bearingsresulting in a lower flux density in the air gap of the magnetic fluidseal. However, a magnetic fluid seal capable of sustaining several mm H₂O of pressure capacity can still be realized.

Additional embodiments of the present invention are illustrated in FIGS.3-9. In these embodiments, the disk drive assembly is the same as theassembly shown in FIG. 2 except for the details of the magnetic fluidseal construction and the requirements for the material of the housing50 and the shaft 42. Accordingly, only the magnetic fluid sealconstruction and the material requirements for housing 50 and shaft 42are described in detail.

With reference to FIG. 3, the annular polepiece 62 is located betweenbearing 52 and the environment to be protected by the seal in the samemanner as described above in connection with FIG. 2. A radiallypolarized annular magnet 70 is attached to shaft 42 in the space 58between bearings 52 and 54. In this embodiment, the shaft 42 is made ofmagnetically permeable material so as to conduct magnetic flux from themagnet 70 through the shaft 42 to the gap between the polepiece 62 andthe shaft 42. The magnetic field retains magnetic fluid 64 in the gap.The housing 50 can be either magnetically permeable or nonmagnetic. Asufficient air gap is maintained between the outer diameter of magnet 70and the inner diameter of housing 50 to divert the magnetic field to theseal formed by polepiece 62 and magnetic fluid 64.

An embodiment of the present invention utilizing an axially polarizedmagnet is shown in FIG. 4. An axially polarized annular magnet 74 islocated in the space 58 between bearings 52 and 54 and is attached tohousing 50. Housing 50 must be made of a nonmagnetic material, at leastin the region adjacent to magnet 74, to prevent magnetic shorting themagnet 74. The annular polepiece 62 and magnetic fluid 64 are locatedbetween bearing assembly 52 and the environment to be protected by theseal as described above in connection with FIG. 2. The magnetic fieldfrom magnet 74 passes through bearing 52 and polepiece 62 and retainsmagnetic fluid 64 in the gap between polepiece 62 and shaft 42. Theshaft 42 is made of a magnetically permeable material to conductmagnetic flux from magnet 74 to polepiece 62 and magnetic fluid 64.

Another embodiment utilizing an axially polarized magnet is shown inFIG. 5. An axially polarized magnet 78 is located in the space 58between bearings 52 and 54 and is attached to the shaft 42. The shaft 42is made of a nonmagnetic material to prevent magnetic shorting of themagnet 78. The annular polepiece 62 and magnetic fluid 64 are locatedbetween bearing 52 and the environment to be protected by the seal asdescribed hereinabove in connection with FIG. 2. The housing 50 is madeof a magnetically permeable material, at least in the region betweenmagnet 78 and polepiece 62 so as to conduct magnetic flux from themagnet 78 to the polepiece 62 and thereby retain the magnetic fluid 64in the gap between the polepiece 62 and the shaft 42.

A bearing and magnetic fluid seal assembly utilizing two magnetic fluidseals is shown in FIG. 6. A radially polarized annular magnet 80 ispositioned in the annular space 58 between bearings 52 and 54 and isattached to housing 50. The polepiece 62 is located between bearing 52and the environment to be protected by the seal as described above inconnection with FIG. 2. The embodiment of FIG. 6 also includes a seconddisk-like annular polepiece 82 located on axis 44 in close proximity tothe bearing 54. Polepiece 82 is on the opposite side of the bearing 54from the magnet 80. The annular magnet 80 is centrally located betweenbearings 52 and 54 and is dimensioned so that its inside diameter isspaced from shaft 42. The housing 50 is made of a magnetically permeablematerial and conducts magnetic flux from magnet 80 to polepieces 62, 82.The shaft 42 can be made of either magnetically permeable or nonmagneticmaterial. Each of the polepieces 62, 82 is attached by a fixed seal atits outside diameter to housing 50 and is in closely-spaced,noncontacting relationship with the shaft 42 so as to define a gapbetween the inside diameter and the shaft 42. A magnetic fluid 64 isretained by the magnetic field in the respective gaps between thepolepieces 62, 82 and the shaft 42. As a result, the embodiment of FIG.6 provides two separate seals along the shft 42.

A variation of the embodiment of FIG. 6 is illustrated in FIG. 7. Thepolepieces 62, 82 are positioned at opposite ends of the bearingassembly as described above in connection with FIG. 6. A radiallypolarized annular magnet 84 is positioned in the space 58 between thebearings 52 and 54 and is attached to the shaft 42. The shaft 42 ismagnetically permeable so that magnetic flux from magnet 84 is conductedthrough the shaft 42 to polepieces 62, 82 to thereby retain the magneticfluid 64 in the gaps between each of the polepieces 62, 82 and the shaft42. Housing 50 can be made of either magnetically permeable ornonmagnetic material. The outside diameter of magnet 84 is dimensionedso that it is spaced from the housing 50.

Embodiments utilizing a pair of annular magnets in the space 58 areillustrated in FIGS. 8-9. In FIG. 8, a first axially polarized annularmagnet 86 is located in the space 58 in close proximity to bearing 54and is attached to housing 50. A second axially polarized annular magnet88 is positioned in the space 58 in close proximity to bearing 52 and isattached to housing 50. The polepieces 62, 82 are positioned at oppositeends of the bearing assembly as described above in connection with FIG.6. The housing 50 is made of nonmagnetic material to avoid magneticshorting of magnets 86, 88. The shaft 42 is made of magneticallypermeable material so that magnetic flux is conducted from magnet 86 topolepiece 82 and from magnet 88 to polepiece 62. The magnetic fluxretains the magnetic fluid 64 in the respective gaps between polepieces62, 82 and shaft 42. The embodiment of FIG. 8 provides one sealcomprising magnet 88 and polepiece 62 and a second seal comprisingmagnet 86 and polepiece 82. The magnets 86, 88 are dimensioned so thattheir inside diameters are spaced from the shaft 42.

FIG. 9 illustrates an alternate embodiment of FIG. 8 wherein an axiallypolarized annular magnet 90 is located in the space 58 between thebearings 52 and 54 and is attached to the shaft 42 in close proximity tothe bearing 52. An axially polarized annular magnet 92 is also locatedin the space 58 in close proximity to the bearing 54 and is attached tothe shaft 42. The shaft 42 must be nonmagnetic to prevent magneticshorting the magnets 90, 92. The housing 50 is of magnetically permeablematerial to conduct magnetic flux from magnet 90 to polepiece 62 andfrom magnet 92 to polepiece 82. The magnetic flux retains the ferrofluid64 in the respective gaps between polepieces 62, 82 and shaft 42.

The embodiments discussed in FIGS. 6-9 are particularly useful for fixedshaft/rotating housing type of disk drive spindles.

The following gives specifications for magnetic fluid suitable for usein each of the above embodiments of the present invention.

    ______________________________________                                        Saturation magnetization:                                                                        100-500 gauss                                              Viscosity:         20-2,000 cp at 27° C.                               Type:              Low vapor pressure                                                            esters, hydrocarbons,                                                         poly ol esters,                                                               fluorocarbons, silicones                                                      and polyphenyl ethers                                      Electrical conductivity:                                                                         Conducting or                                                                 nonconducting                                              ______________________________________                                    

The polepieces 62, 82 are made of magnetically permeable material suchas 400-Series stainless steel, have an axial dimension in the range of0.5 mm to 1.5 mm and define a radial gap between the polepiece insidediameter and the associated shaft in the range of 0.07 mm to 0.25 mm.The magnets utilized in the embodiments of FIGS. 2-9 are polarized asspecified and can be ferrites, Alnico, Sm-Co or Nd-Fe-B with an energyproduct in the range from 1.1×10⁶ to 10×10⁶ gauss-oersted.

EXAMPLE

A bearing and magnetic fluid seal assembly was constructed in accordancewith the embodiment of FIG. 3. The spindle housing was nonmagnetic andthe bearings were made of magnetic material. The spindle shaft had adiameter of 7 mm and the polepiece had an axial dimension of 1 mm. Theradial gap between the polepiece inside diameter and the shaft was 0.20mm. The polepiece inside diameter was 7.4 mm while the polepiece outsidediameter was 19 mm. The magnet was rigid ferrite radially polarized,having an energy product of 1.6×10⁶ gauss-oersted. The magnet length was7.6 mm. The magnet inside diameter was 7 mm and the magnet outsidediameter was 16.5 mm. The magnetic fluid type was 450 gauss, 300 cp at27° C. and was ester based. The seal was tested for its pressurecapacity which was found to be 250 mm H₂ O.

While there has been shown and described what is at present consideredthe preferred embodiments of the present invention, it will be obviousto those skilled in the art that various changes and modifications maybe made therein without departing from the scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A magnetic fluid seal assembly comprising:ahousing; a shaft having an axis of rotation; a bearing assembly forsupporting said shaft and for permitting rotation of said shaft relativeto said housing, said bearing assembly including at least two axiallyspaced-apart bearings between said shaft and said housing; annularmagnet means axially positioned between said bearings for producing amagnetic field; polepiece means including an annular polepiece axiallypositioned outside said bearing assembly and extending inwardly fromsaid housing toward said shaft, said polepiece having a closely-spaced,noncontacting relationship with said shaft, thereby defining a gapbetween said polepiece and said shaft, at least a portion of saidmagnetic field existing in said gap, said annular polepiece being spacedfrom said annular magnet means by one of said bearings, said polepiecemeans being located entirely outside said bearing assembly so as to forman open magnetic circuit wherein said annular magnet means is not incontact with said polepiece means; and a magnetic fluid retained by saidmagnetic field in the gap between said polepiece and said shaft, therebyproviding on at least one end of said bearing assembly a low pressurecapacity magnetic shaft seal having a short axial dimension.
 2. A sealassembly as defined in claim 1 wherein said polepiece means includesannular polepieces axially positioned on opposite sides of said bearingassembly and extending inwardly from said housing toward said shaft,said polepieces each having a closely-spaced, noncontacting relationshipwith said shaft, thereby defining a gap between said polepiece and saidshaft.
 3. A seal assembly as defined in claim 1 or 2 wherein saidannular magnet means comprises a radially polarized magnet attached tosaid housing and wherein said housing between said magnet and saidpolepiece is magnetically permeable.
 4. A seal assembly as defined inclaim 1 or 2 wherein said annular magnet means comprises a radiallypolarized magnet attached to said shaft and wherein said shaft betweensaid magnet and said polepiece is magnetically permeable.
 5. A sealassembly as defined in claim 1 or 2 wherein said annular magnet meanscomprises an axially polarized magnet attached to said housing, whereinsaid shaft is magnetically permeable and wherein said housing isnonmagnetic.
 6. A seal asembly as defined in claim 1 or 2 wherein saidannular magnet means comprises an axially polarized magnet attached tosaid shaft, wherein said shaft is nonmagnetic and wherein said housingis magnetically permeable.
 7. A seal assembly as defined in claim 2wherein said annular magnet means comprises a pair of spaced-apartaxially polarized magnets.
 8. A seal assembly as defined in claim 7wherein said axially polarized magnets are attached to said housing,wherein said shaft is magnetically permeable and wherein said housing isnonmagnetic.
 9. A seal assembly as defined in claim 7 wherein saidaxially polarized magnets are attached to said shaft, wherein said shaftis nonmagnetic and wherein said housing is magnetically permeable.
 10. Aseal assembly as defined in claim 1 or 2 wherein said magnetic fluid iselectrically conductive.
 11. A seal assembly as defined in claim 1 or 2wherein each annular polepiece comprises a thin annular disk ofmagnetically permeable material.
 12. A seal assembly as defined in claim11 wherein each annular polepiece has an axial dimension in the rangebetween 0.5 mm and 1.5 mm.
 13. A seal assembly as defined in claim 1 or2 wherein each bearng is magnetically permeable.
 14. A seal assembly asdefined in claim 1 wherein each bearing is nonmagnetic.